Exercise is a bit like coriander: some people love it, others hate it. But what explains the gap between those who are afraid of the gym and those who are afraid of missing even one session there? A new study of brain signaling networks in the journal Medicine & Science in Sports & Exercise offers an optimistic outlook for bridging this gap. Follow your workout routine during these initially unpleasant weeks, suggest results, and you can also learn to love the gym, thanks to long-term adjustments to the way your brain processes mood-altering chemicals. Previous studies have shown that regular exercisers tend to have lower levels of anxiety and depression compared to less active people, and also have a greater mood boost after a single workout. It’s a cycle that strengthens you: you are more likely to be motivated for your next workout if the last one made you feel great. But it is not clear how this cycle begins or what changes in brain chemistry make it possible. To find out, researchers at the University of Turkey in Finland recruited 64 volunteers to complete a series of exercise tests and questionnaires. They used a medical imaging technique called positron emission tomography to measure the activity of mu-opioid receptors or MORs in the brain. These MORs respond to the presence of endogenous opioids, the body’s own version of opioids, and play a role in the processing of reward, pain, motivation, stress, and emotions. “It is possible for some people to be born with a more sensitive MOR system and it helps them to tolerate and enjoy exercise, so it is easy for them to exercise at a higher level,” explains Tiina Saanijoki, lead author of the study. . . “Or it could be the other way around, so a MOR system has been developed that works best through regular exercise habits.” Saanijoki put her volunteers to a test in which they cycled to the point of exhaustion to assess their aerobic fitness. Another day, some of them did an hour of continuous moderate cycling and others did a high-intensity cycling session to determine how different types of exercise affected opioid signaling. Certainly, the strongest individuals (as measured by both the cycling test and the weekly exercise levels reported by themselves) saw the greatest change in MOR activity after continuous moderate training. The same was true for the heaviest trainees after the high-intensity break training. The more exercise you do, it seems, the more neurochemically satisfying it is. This still does not prove that regular exercise leads to a more sensitive opioid system, as opposed to the other way around. But studies in rats offer some clues to the previous explanation. For example, rats exercising for five to eight weeks have higher levels of endogenous opioids such as endorphins circulating in their brains. There is an important caveat here. The discussion about opioids and endorphins may suggest that exercise causes some kind of euphoria in the respondents. This is the impression made by early research on what is known as a high runner in the 1970s, but later studies have found that such experiences are extremely rare. “Runner’s high is quite mythical and may not be worth focusing on too much,” says Saanijoki. In contrast, the senses and emotions caused by exercise tend to be subtle, sometimes operating below the level of consciousness, and are probably mediated by many different sets of brain chemicals besides opioids, including endocannabinoids, cannabis in the brain. However, the findings suggest two key solutions. The first is that if you do not enjoy the exercise, it does not necessarily mean that you are doing something wrong. “I think we have to recognize that there is a huge variety of people in these responses and not all people find exercise enjoyable or rewarding,” says Saanijoki. The second is that this can change. As your body adjusts to a new exercise routine, so does your brain – and if Saanijoki’s hypothesis is correct, you may end up wondering how you ever lived without it. Alex Hutchinson is its author Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience. Register for the weekly Health & Wellness newsletter for the latest news and tips.
